1. Field of the Invention
This invention relates to the field of oxygen enrichment in fuel conveying gases for use in combustion.
2. Related Art
Oxygen enrichment in all kinds of combustion systems is a well known and growing method already implemented in many industrial processes to improve combustion characteristics, both in terms of efficiency, product quality and pollutant emission levels. These include glass furnaces, cement and lime kilns, and aluminum or steel processes. For example, oxygen enrichment in lime kilns has been described in Garrido G. F., Perkins A. S., Ayton J. R., UPGRADING LIME RECOVERY WITH O2 ENRICHMENT, CPPA Conference, Montreal, January 1981. Pure or substantially pure oxygen may be used as the only oxidant for some specific applications, often referred to as “full oxygen” furnaces. It can also be effective when added as a complementary oxidant in an existing air-fired combustion system, either through the ports enabling the air stream to flow into the combustor or through additional ports added for pure oxygen or oxygen-enriched air injection. This last case is often referred to as “oxygen-enriched” combustion or “oxygen-enhanced” combustion
Two principal alternatives can be implemented for oxygen enrichment, either premixing the oxygen, usually in at least some of the existing air to form an oxygen-enriched oxidant stream, or injecting the oxygen directly into the combustion chamber. Premixing can be achieved by injecting the O2 in some of the main air-ducts, to produce a homogeneous, oxygen enriched stream for introduction into the combustion chamber. Direct injection can be achieved through substantially pure O2 lancing into the combustion chamber, through specific ports apart from existing air ports, or through existing air ports, the oxygen lances being surrounded by the main air stream without mixing into this air stream before the exit to the combustion chamber.
The furnaces thus described that employ pure O2 streams or oxygen enriched streams operate with gaseous or liquid fuels such as natural gas or oil. In those cases, all oxidant streams can be categorized as “solely oxidant” streams, since their only role consists in providing the oxidant (the oxygen molecules needed for combustion) to the combustion zone. To date, none of these oxygen-enrichment schemes has been successfully adapted to solid-fuel applications, such as pulverized coal-fired boilers, due to problems associated therewith that are unique to solid-fuel media and their transportation, as described below.
Gas- or oil-fired furnaces usually require only two types of air streams. The first of these types is typically positioned at the burner level, and can comprise as much as 100 percent of the air required for complete combustion. The second type, if necessary, is positioned apart from the burner, and is injected in a “second combustion zone” to complete the combustion.
In the case of pulverized coal-fired boilers, and other devices where solid fuel particles (these can include any apparatus for burning a solid that is pulverized, micronized or otherwise exists in a fine enough state to be transported by a gas flow) require a conveying gas to transport it to the burner, the first oxidant-stream contacting the fuel in a “first combustion zone” consists of the conveying gas itself, typically air. This air stream conveying the solid fuel particles from a fuel storage or milling device (e.g., a coal pulverizer) to the burner is often referred to as “primary air,” and corresponds to about ten to twenty percent of the overall air injected into the combustion chamber to effect complete combustion of the fuel. Note that its function is more than that of the “solely oxidant” air stream described above; its primary function is to convey the fuel to the burner. Indeed, it need not be an oxidant at all—it could be a gaseous fuel, such as natural gas, or an inert gas, such as nitrogen. Currently, it is often pragmatic to use air as the conveying gas. In any event, it is desirable that, regardless of the oxidant characteristics of this gas, it have sufficient volume and flowrate to accomplish the transportation of the solid fuel to the burner.
These conventional, pulverized coal-fired boilers use at least two, and sometimes three, types of air streams. Note that there can be multiple streams of each type in use, depending on the specific design of the structure. The first of these is the primary air stream, conveying the pulverized solid fuel. The second type, “secondary air,” is injected at the burner level, around or near the primary air/fuel mixture. The third type, referred to as “tertiary air” or “overfire air (OFA),” is injected, if necessary, outside the burner in a second combustion zone, to complete the combustion process. This conventional coal-fired boiler is illustrated in FIG. 1.
Some studies reported in the literature show that increasing the temperature in the fuel rich ignition zone would allow a quicker and more efficient release of volatiles contained in the pulverized fuel, thus increasing the flame stability, enhancing the combustion efficiency, enabling an easier operation and saving fuel. It would also decrease the pollutant emissions, especially NOx formation, since fuel-rich combustion coupled with high temperatures is known to prevent fuel-bound nitrogen from being oxidized to nitrogen oxides, by reducing it to molecular nitrogen (N2). This is more fully described, for example, in Sarofin, A. F. et al., “Strategies for Controlling Nitrogen Oxide Emissions during Combustion of Nitrogen-bearing fuels”, PROCEEDINGS OF THE 69TH ANNUAL MEETING OF THE A.I.CH.E., Chicago, November 1976, as well as in K. Moore, W. Ellison, “Fuel Rich Combustion, A Low Cost NOx Control Means for Coal-fired Plants,” 25TH INTERNATIONAL TECHNICAL CONFERENCE ON COAL UTILIZATION & FUEL SYSTEMS, Clearwater, Fla., March 2000. To increase the temperature in the combustion, a well known process is to increase the local oxygen content, or in other words to release more energy per unit of volume (fuel/oxidant/flue gas volume). Oxygen-enrichment in the fuel-rich ignition zone will then help increase the local temperature and get the related benefits previously described. As the first air stream in contact with the fuel and as the only oxidant stream available in the very beginning of the combustion process, the primary air may seem to be suitable to get higher O2 content in the ignition zone.
While it appears then theoretically desirable to enrich the primary air to increase the temperature in the fuel-rich ignition zone, two problems have in the past prevented adaptability of known techniques already used or described for secondary or tertiary air enrichment. First, the primary air, as opposed to all other oxidant streams, contains fuel particles. The existing fuel/primary air stream is then a flammable gas, which will become even more flammable if oxygen is injected into it. Oxygen-injection into the fuel conveying primary air must be handled with great care. Second, oxygen-enrichment of the primary air by replacing a portion of it (the function of which includes transporting the pulverized fuel) with the stoichiometric equivalent of oxygen would reduce the volume of the conveying gas and may adversely affect the characteristics of the fuel-carrying gas stream.
Thus, a problem associated with coal-fired burners and other pulverized solid-fuel, air-fired combustion systems that precede the present invention is that they produce an level of NOx emission that is unacceptable in view of existing environmental regulations.
Yet another problem associated with pulverized coal-fired burners and other pulverized solid-fuel burners that precede the present invention is that they are not susceptible to traditional oxygen enrichment techniques upstream from the point of ignition, as they would then bear an unacceptably high risk of premature ignition, explosion, or other detrimental effects.
Still another problem associated with pulverized coal-fired burners and other pulverized solid-fuel burners that precede the present invention is that they have not been successively modified to provide adequate combustion characteristics resulting in adequate reduction of NOx formation sufficient to meet environmental guidelines without expensive and complex NOx treatment apparatus.
Another problem associated with pulverized coal-fired burners and other pulverized solid-fuel burners that precede the present invention is that they have not been adaptable to oxygen enrichment that facilitates NOx reduction while at the same time permits a maintained flow of a conveying gas to facilitate flow of the pulverized fuel from storage to the burner.
An even further problem associated with pulverized coal-fired burners and other pulverized solid-fuel burners that precede the present invention is that they have not been provided with a multiplicity of oxygen enrichment tools that permit substantial reduction of NOx with the least amount of oxygen necessary.
Another problem associated with pulverized coal-fired burners and other pulverized solid-fuel burners that precede the present invention is that they have not been provided with a multiplicity of oxygen distribution variables so as to be retrofittable to provide optimal reduction of NOx with the least amount of oxygen.
For the foregoing reasons, there has been defined a long felt and unsolved need for a pulverized coal-fired burner or other pulverized solid-fuel burners that facilitates oxygen enrichment therein to effectively reduce NOx production while at the same time maintaining the operability and safety of the burner process.